Such a material with high strength as 18% Ni-maraging steel has been used so far for a continuously variable transmission belt. A metastable austenitic stainless steel is sometimes used for the purpose, as disclosed in JP 2000-63998A. The continuously variable transmission belt is conventionally manufactured by the following steps: A steel strip is formed to a ring shape by plasma- or laser-welding its front and tail ends together. The welded steel strip is heat-treated to eliminate a hardness difference between base and welded parts and smoothened at its edge by barreling. The steel strip is then ring-rolled to a predetermined thickness and stretched to a predetermined circumferential length. Thereafter, the steel strip is nitrided and aged so as to harden its surface layer.
The manufactured steel belt is subjected to a rotation-tensile fatigue test or the like for evaluation of fatigue properties. 18% Ni-maraging steel, which is strengthened by work-hardening and aging (strain-aging), has excellent fatigue properties due to a hard nitrided surface layer and effects of cold-working on mechanical properties. However, 18% Ni-maraging steel is scarcely work-hardened due to its large deformation resistance, so as not to anticipate an increase of strength derived from work-hardening even by ring-rolling with a heavy duty. The heavy-duty rolling often causes damages of a steel strip during rolling, when the steel strip lacks of ductility.
A metastable austenitic stainless steel is also a kind of steel, which is work-hardened or strain-aged by cold-rolling. Its strength is remarkably improved by formation of strain-induced martensite and work-hardening of residual austenite in comparison with 18% Ni-maraging steel, but its strengthening rate is varied in correspondence to a material temperature during rolling. Heat generation and dissipation during rolling put significant effects on mechanical properties of a rolled steel strip or belt. In this consequence, a steel belt manufactured by ring-rolling has thickness, width and cross-sectional hardness deviated in response to a manufacturing season.
In short, it is difficult to manufacture a steel belt, which has stable material strength necessary for use as a continuously variable transmission belt. The difficulty is somewhat caused by mechanical properties of the metastable austenitic stainless steel.